The present invention relates generally to an apparatus and a method for measuring three-dimensional shape of a generally cylindrical wood block. More specifically, the invention relates to an apparatus and a method for measuring the distances of the wood block from a tentative spin axis on which the block is rotatably supported to the outer peripheral surface of the block at a plurality of angular positions thereof thereby to determine profiles of the block in a plurality of cross-sections thereof perpendicular to and spaced along the tentative spin axis of the block.
Wood veneer which is used for manufacturing various kinds of wood products such as plywood is cut or peeled from a generally cylindrical wood block by a veneer knife which is fed into the block from its peripheral surface toward the spin axis of the block. Such cutting or peeling is done by a machine called veneer lathe. In a veneer lathe, a wood block is rotatably supported by spindles having at the inner ends thereof chucks engaging with the block at the opposite ends thereof. In cutting veneer from wood block, the veneer knife is fed into the rotating block toward its spin axis for a predetermined distance for each rotation of the block thereby to peel a sheet of veneer having a predetermined thickness
As is well known to those skilled in the art, a block that is made by cross-cutting a natural log to any desired length does not have a true cylindrical surface. For producing veneer from such a block with as high a yield rate as possible, three-dimensional shape of the block should be determined and known previously and the optimum spin axis on which the block is to be rotated for the best yield rate should be figured out. In initiating the peeling operation, the knife carriage of the veneer lathe carrying the veneer knife must be spaced away from the block so that no damaging interference occurs between the knife and the block when it just starts to rotate.
If the veneer knife just before being moved toward the block is spaced for a long distance from the spin axis of the block, it takes a long time accordingly before the knife engages with the block surface to start veneer peeling. The time spent before actual peeling begins is a downtime affecting the productivity of the veneer lathe. In order to minimize this downtime, the point on the block peripheral surface having the largest distance as measured from the spin axis should be located and the value of the largest distance should be figured out previously. Veneer knife on the knife carriage may be set as close to the block peripheral surface as possible based on the value of the largest distance. By so setting the veneer knife, the distance for which the knife has to move without cutting can be minimized and the working efficiency of the veneer lathe can be improved, accordingly.
Thus, the optimum spin axis and the point on the block having the largest distance between the spin axis and the outer peripheral surface of the block should be determined for improving the yield in veneer peeling and the working efficiency of the veneer lathe. For finding the optimum spin axis and the largest distance, three-dimensional shape of the wood block should be measured. Lathe charger is a machine that performs the functions of measuring the three-dimensional shape of a block, determining the supporting position of the block by the lathe spindles and also the stand-by position of the knife carriage relative to the block and then charging the veneer lathe with the peeler block.
Lather chargers that are designed to measure three-dimensional shape of a wood block using mechanical contact type sensors or non-contact type sensors such as ultrasonic sensors or laser interferometer are disclosed, for example, by Japanese Patent Application Publications Nos. H06-293002 and 2007-90519, corresponding to U.S. Pat. Nos. 5,449,030 and 7,275,571, respectively.
The lathe charger of the Publication No. H06-293002 has a plurality of contact type sensors arranged along the entire length of a wood block in close proximity to each other in facing relation to the outer peripheral surface of the block and as many displacement detectors as the sensors for measuring the displacement of their corresponding sensors. According to this lathe charger, with the sensors set in contact with the outer peripheral surface of the block, the block is rotated on a tentative spin axis for a complete turn and the amount of displacement of each sensor is measured by its displacement detector. Thus, profiles of the block in cross-section perpendicular to the tentative spin axis at a plurality of positions along the block length are determined. It is noted that the tentative spin axis is an axis which is selected tentatively and on which the block is rotated for finding the three-dimensional shape of the block and then determining an optimum spin axis of the block. The Publication also proposes an apparatus wherein the above contact type sensors are replaced by non-contact type profile detectors arranged in close proximity to each other along the length of the block.
The lathe charger according to the Publication No. 2007-90519 has a plurality of non-contact type distance detectors arranged along the length of a wood block for determining the distances from the respective distance detectors to the outer peripheral surface of the block at each of the predetermined angular positions of the block about a tentative spin axis of the block. The apparatus further has a plurality of swing arms juxtaposed along the length of the block and set in contact with the outer peripheral surface of the block and an angle detectors disposed for each swing arm for measuring the angle for which the arm has swung in accordance with the rotation of the block. The optimum spin axis on which the block should be rotated in actual veneer peeling is found based on the data of distances detected by the distance detectors, and the maximum radius point on the block is determined based on the data of angles of rotation of the swing arms detected by the angle detectors.
According to the apparatuses of the above Publications, however, the resolution in measurement of three-dimensional shape of a wood block (or profiles of the block in a plurality of sections across the tentative spin axis of the block) is poor in that the resolution is limited by the number of sensors or detector arranged along the block length. Specifically, any defective part or irregularities of a wood block such as knot, burl, hole, recess, etc. present between any two adjacent sensors or detectors cannot not be recognized.
When contact type sensors are used in measuring three-dimensional shape of a wood block having various irregularities such as projection on the peripheral surface thereof, each sensor can only detect the highest point of a projection in its detecting region without determining accurately the profile of the other parts than the highest point. In the apparatus using non-contact type sensors, the distances can be determined only at the positions where the sensors are provided and the distances at other positions of the block cannot be measured. Incapability of accurately measurement of three-dimensional profile of a wood block throughout its length makes it difficult to find an accurate position of the optimum spin axis of the block and a point on outer peripheral surface of the block having the largest distance from the spin axis that are necessary for improvement of yield and working efficiency in veneer peeling.
The present invention, which has been made in light of above problems in conventional apparatuses, is directed to providing an apparatus and a method for measuring accurately three-dimensional shape of a wood block or profiles of the block in sections across its tentative spin axis along the entire block length.